The small intestine is the primary organ of the digestive tract. In the small intestine, chyme is finally mixed for its passage to cells. The small intestine may be subdivided into three segments. A sphincter is located at the top of the small intestine. The first segment of the small intestine is known as the deudodenum. The deudodenum is about 25 cm (10 inches). The second segment of the small intestine is known as the jejunum. The jejunum is about 2.5 meters long, (8 feet). The final segment of the small intestine is known as the ileum. The ileum is about 3.6m (12 feet). Most of food digestion through acids and bases occurs in the small intestine.

Chemical digestion reaches its " full swing " within the small intestine. Though the small intestine is mostly responsible for digestion, it can only process a small amount of chyme at a time. The pyloric sphincter acts as a gatekeeper for the small intestine; pyloric sphincter literally means gatekeeper. The pyloric sphincter is located at the entrance to the small intestine from the stomach. There are a number of enzymes, which are produces within intestinal cells as well as the pancreas and liver. Bile, an enzyme, helps digest the chyme furthermore, and is produced within the liver. Bile enters the small intestine through the bile duct. The hepatopancreatic ampulla is the region of the duodenum in which the pancreatic as well as the bile ducts converge together.

  Most of food absorption is processed by the small intestine. The small intestine has three structures to handle absorption, villi, microvilli and the circular folds. The purpose of villi, microvilli and the circular folds is to increase the area of absorption within the small intestine. Microvilli are small projections of the mucous membrane of the small intestine walls. The microvilli give the small intestine walls a fuzzy appearance and are also know as the brush border. The structure of Villi is very similar to the microvilli except that villi are substantially larger than microvilli. Within all villi are a large number of capillaries and a lymphatic capillary as well.

    In addition to digesting chyme, the small intestine is responsible for absorbing the processed food into the bloodstream, so that energy can be derived from the food. This absorption takes place through anatomical structures known as Villi. Villi are structures, which act as valves, which allow solids liquids and/or acids to pass through one structure to another. Through these Villi, all the useful nutrients that are located in chyme pass through the small intestine into the bloodstream. The undigested chyme or not useful nutrients then pass to the large intestine.

The large intestine is about 1.5 meters (5 feet). The large intestine is also called the colon. The large intestine stretches from a valve that rests between the bottom of the small intestine and the top of the large intestine, to the anus. The main job of the small intestine is to digest any undigested chyme, and to rid the body of any unneeded wastes by sending it to the anus. The large intestine has multiple subdivision such as the cecum, the appendix, the colon, the rectum, and the canal. The first section of the large intestine is the cecum. The appendix hangs from the cecum. The colon is subdivided into a number of regions. The ascending colon travels in an upward direction of the abdominal cavity. The descending colon travels in a downward fashion toward the pelvis. Other regions of the colon include transverse colon and the sigmoid colon. The rectum lies within the pelvis. It is the last subdivision of the large intestine before the anus. The anus is where the canal comes to an end. The anal canal contains a voluntary sphincter. The voluntary sphincter is composed of skeletal muscle. The canal also contains an involuntary sphincter which is composed of smooth muscle.